Improved melt strength in combination with superior processability has been a long-standing interest in the polypropylene (PP) industry. Some attempts to achieve this result include the use of bimodal PP blends, either in situ (in reactor) or ex situ (extruder blends), which include a combination of a relatively low molecular weight polypropylene (or high MFR) and a relatively high molecular weight polypropylene (or low MFR). The challenge is particularly relevant for blends made from single-site catalysts, which alone are not easily processable due to narrow molecular weight distribution.
Use of a high molecular weight PP may improve the melt strength of polypropylene blends. However, inclusion of HMW PP in blends may be accompanied by a loss in the processability due to increased viscosity if the properties of the components are not properly balanced. Depending on the ultimate application, solutions to achieve a reasonable result between melt strength and processability may involve adjusting the properties of the individual polypropylene components and/or addition of some other polymeric additive such as an elastomeric component, either of which may be accompanied by forming bimodal PP blends. While in-reactor methodologies to form bimodal PP blends are well documented, physical blending of two unimodal polypropylenes to realize bimodality has not shown improved melt strength without sacrificing processability.
The present invention describes new bimodal PP compositions prepared by melt blending exhibiting a good balance of melt strength and processability.